This invention relates generally to wastewater treatment and, more particularly, to a composition and method for simultaneously precipitating metal ions from semiconductor wastewater and enhancing microfilter operation.
Until the development of copper interconnect technology, copper was not found in wastewater from the production of multilayer microchips by the semiconductor industry. Copper is now being used as a replacement for aluminum and tungsten because of its lower electrical resistivity. In the process of manufacturing multilayer chips, there are many steps including the deposition of the dielectric layer (silicon dioxide or low-k polymeric), etching the interconnect pattern (trenches and vias) into the dielectric layer, deposition of copper metal into the trenches and vias, and chemical mechanical polishing (CMP) to remove excess copper and create a level surface prior to creation of the next layer of the chip. The CMP step is accomplished by the use of polishing pads and proprietary polishing slurries which contain abrasive solids such as alumina, oxidants such as peroxide, chelants such as citrate, and other additives such as corrosion inhibitors. Therefore, the resulting wastewater contains chelated copper, oxidants, additives and abrasive solids. The presence of the abrasive solids at concentrations of 200-5000 mg/L makes this wastewater different than the typical metal-containing wastewater from electroplating operations.
Wastewater, from the metal-CMP step of microchip manufacture, can vary widely depending on the original slurry composition and the CMP tool design and operating parameters. The slurry is diluted by rinse water during polishing. The amount of rinse water used determines metal and abrasive solids levels in the wastewater.
Several polymer chemistries have been used to treat wastewater containing transition metal complexes, such as copper-ethylenediaminetetraacetic acid (EDTA), resulting in the precipitation of the metal-polymer solids. These polymer chemistries contain an amine functionality that can be reacted with carbon disulfide to form dithiocarbamate (DTC) functionalities on a polymer backbone. One such polymer is a carbon disulfide modified ethylenedichloride-ammonia condensation polymer, as described in U.S. Pat. No. 5,164,095. The polymer disclosed in the ""095 patent is a low molecular weight, highly branched material. Other polymer backbones suitable for modification with carbon disulfide include the polyethylenimine (PEI) polymer described in U.S. Pat. No. 5,387,365, the epichlorohydrin and multifunctional amine condensation polymer disclosed in U.S. Pat. Nos. 4,670,160 and 5,500,133 and the polyallylamine polymer taught in EP 0 581 164 Al. Despite these known polymer chemistries, however, there is still a need for new polymers containing DTC functionalities which effectively treat wastewater and possess other desirable attributes such as low levels of product impurities (e.g., sodium sulfide, which is toxic and foul smelling), relative ease of manufacture (e.g., to avoid the gaseous ammonia in ethylenedichloride and azeridine in PEI) and improved solids/liquid separation characteristics.
U.S. Pat. No. 5,346,627 describes the use of polymers containing DTC functionalities for the treatment of soluble metals and subsequent removal of precipitated solids in a filtering device, including a microfilter. However, the ""627 patent does not describe the use of such polymers for simultaneously precipitating metal ions from semiconductor wastewater and enhancing microfilter operation.
There are two parameters of concern during microfilter operation. One is flux which is defined as the flow of purified water divided by the membrane area. In microfiltration, one way of expressing this is as gallons of pure water per square foot of membrane area per day or GFD. Another way of expressing this is permeability, which is flux divided by trans membrane pressure (TMP). Permeability is essentially xe2x80x9cnormalizedxe2x80x9d flux which takes into account changes in system pressures. Both flux and permeability are used to describe the passage of water through the membrane, however, they are not interchangeable. The other parameter of concern is solids passage. Generally speaking, the purpose of the microfilter is to separate solids from liquids in a bulk solution. Because the microfilter has a distinct cutoff size (ranging from approximately 0.1 to 5.0 microns depending on method of manufacture), only particles larger than the cutoff are retained, in theory. However, as is the case with all membrane processes, a percentage of the total solids will pass through the membrane. Therefore, as the initial concentration of the feed water increases, so too will the absolute value of the solids content of the permeate water. The percentage of materials which pass through the filter, however, remains largely the same unless membrane damage has occurred.
In many operations, microfiltration is used to perform the functions of a clarifier and a media filter. This is because it has a small footprint and performs these operations more quickly than conventional technology. It is important, therefore, to keep the microfilter in good operating condition. A major problem with microfiltration is that the filters can become fouled or plugged with fine solids. This causes the flux to decrease in the unit, and it must be taken off line to be cleaned. Microfilter operation can be enhanced by using additives which result in higher flux values and longer times between occurrences of fouling. Flux enhancement is desirable because it decreases the amount of time during which the equipment is out of service, thereby increasing its overall efficiency.
Accordingly, it would be desirable to provide a new water-soluble polymer containing DTC functionalities which effectively treats metals-contaminated wastewater and possesses other desirable properties, such as a low level of product impurities, relative ease of manufacture and improved solids/liquid separation characteristics. It would also be desirable to provide a method for the use of such a polymer to simultaneously precipitate soluble heavy metal ions from semiconductor wastewater containing abrasive solids and enhance microfilter operation.
The present invention is directed to a new water-soluble polymer containing DTC functionalities and the use of such a polymer in a method for simultaneously precipitating soluble heavy metal ions from semiconductor wastewater containing abrasive solids and enhancing the operation of a microfilter.
The new water-soluble polymer containing DTC functionalities effectively treats semiconductor wastewater, contains a low level of product impurities, can be relatively easily manufactured and exhibits improved solids/liquid separation characteristics. When an effective amount of this or a similar polymer containing DTC functionalities is added to semiconductor wastewater, the simultaneous precipitation of soluble heavy metal ions from the wastewater and enhancement of microfilter operation is achieved.
The present invention is directed to a composition and method for simultaneously precipitating soluble heavy metal ions from semiconductor wastewater containing abrasive solids and enhancing the operation of a microfilter.
The composition of the present invention is a water-soluble polymer which contains DTC functionalities and has the formula: 
wherein R is H or CSxe2x88x922X+and X+is an alkali metal (such as sodium or potassium), an alkaline earth metal or ammonium, and n is the number of repeating units such that the polymer has a total molecular weight in the range of from about 3000 to about 100,000.
The new composition is prepared by polymerizing diallylamine and then subsequently reacting it with CS2 to form DTC functionalities on the polymer backbone. The diallylamine may also be copolymerized with suitable monomers. For example, the suitable monomers may have an anionic, cationic or neutral charge and may include acrylate, acrylamide or vinyl containing monomers. The synthesis is shown below: 
The amount of DTC functionalization (i.e., the percent of R as CSxe2x88x922X+) on the polymer can be controlled and should be at least about 5%, and preferably from about 20 to about 70%. The resulting water-soluble polymer containing DTC functionalities is more linear, i.e., less branched, than other known DTC-polymers using a polyethylenimine type backbone and contains a cyclic amine. Also, the inventive polymer contains only secondary amine groups in the polymer backbone, precluding the formation of unstable sodium dithiocarbamates and the subsequent degradation to sodium sulfide.
In accordance with the method of the present invention, either the new or a known water-soluble polymer containing DTC functionalities is used to simultaneously precipitate soluble heavy metal ions from semiconductor wastewater containing abrasive solids and enhance the operation of a microfilter.
The water-soluble polymer contains DTC functionalities and has the formula: Rxe2x80x22Nxe2x80x94R wherein Rxe2x80x2 is an alkyl group, aryl group or substituted derivative thereof and R is H or CSxe2x88x922X+and X+ is an alkali metal (such as sodium or potassium), an alkaline earth metal or ammonium. One skilled in the art would recognize that Rxe2x80x22Nxe2x80x94R is a repeating unit in the polymer. The water-soluble polymers which may be used in the practice of this invention preferably include the dithiocarbamate derivatives of polydiallylamine, polyethylenimine and the epichlorohydrin and multifunctional amine condensation polymer of U.S. Pat. No. 5,387,365, the disclosure of which is incorporated herein by reference. As used herein, xe2x80x9cpolyethyleneiminexe2x80x9d is meant to include the condensation polymer prepared from ethylenedichloride and ammonia. Any suitable polymer containing primary or secondary amine groups which can be modified to form dithiocarbamate functionalities may also be used in the practice of the present invention.
The soluble heavy metal ions which may be precipitated from the semiconductor wastewater include copper, nickel, zinc, lead, mercury, cadmium, silver, iron, manganese and mixtures thereof.
The abrasive solids which may be present in the semiconductor wastewater include alumina, silica, ceria, germania, titania, zirconia and mixtures thereof.
Oxidants may also be present in the semiconductor wastewater. These oxidants include potassium iodate, potassium ferricyanide, hydrogen peroxide, ferric nitrate, silver nitrate, nitric acid, sulfuric acid, potassium hypochlorite, potassium permanganate, ammonium persulfate, ammonium peroxydisulfate, peracetic acid, periodic acid, peroxymonosulfuric acid, potassium peroxomonosulfate, peroxymonosulfate, malonamide, urea-hydrogen peroxide, potassium dichromate, potassium bromate, vanadium trioxide, oxygenated water, ozonated water and mixtures thereof.
Although it is not a requirement of the present invention, the oxidants may optionally be removed from the wastewater. Ways to remove these oxidants are generally known to those skilled in the art (See e.g., U.S. Pat. No. 5,464,605). However, if the oxidants are not removed from the wastewater or if any oxidants remain, they may be reduced by the method of the present invention.
The water-soluble polymer containing DTC functionalities is dosed at an amount which will effectively precipitate soluble heavy metal ions from semiconductor wastewater containing abrasive solids and simultaneously enhance microfilter operation. It is preferred that the DTC-polymer be added to the wastewater in at least an amount effective for precipitating the heavy metal ions from the wastewater. The addition of such an effective amount will simultaneously enhance microfilter operation. An amount greater than the metal precipitating amount can be added to the wastewater if it is desirable to further increase the flux of permeate through the microfilter. The adjustment of dosage can be made manually by one skilled in the art of microfilter operation or automatically by the use of polymer-specific sensors.
The DTC-polymer can be added to the semiconductor wastewater by any conventional method, preferably before the microfilter. Also, prior to the addition of the DTC-polymer to the semiconductor wastewater, the pH of the wastewater may preferably be adjusted to between 4 and 12. More preferably, the pH may be adjusted to between 6 and 10, with between 7 and 9 being most preferred.
The types of microfilters suitable for use in the practice of the present invention are generally known to those skilled in the art. Such microfilters include ceramic membrane and synthetic membrane units, such as the U.S. Filter Membralox(copyright) and the U.S. Filter Memtek units, respectively. It is preferred that the microfilter have a molecular cutoff size in the range of about 0.1 to about 5 microns and, more preferably, in the range of about 0.1 to about 1 micron.
The present inventors have discovered a new water-soluble polymer containing DTC functionalities which effectively treats semiconductor wastewater, contains a low level of product impurities, can be relatively easily manufactured and exhibits improved solids/liquid separation characteristics. Moreover, it has been discovered that when an effective amount of the inventive or a similar polymer containing DTC functionalities is added to semiconductor wastewater, the simultaneous precipitation of soluble heavy metal ions from the wastewater and enhancement of microfilter operation is achieved. This is surprising because polymers containing DTC functionalities are negatively charged at the treatment pH and cationic polymers are typically used in such applications for flux enhancement. (See Water Treatment Membrane Processes, American Water Works Association Research Foundation, Lyonnaise des Eaux, Water Research Commission of South Africa, McGraw-Hill, 1996, Chap. 16, Coagulation and Membrane Separation.)
The use of small molecule precipitant sodium dimethyldithiocarbamate (DMDTC) with cationic coagulants is known to those skilled in the art for metal ion precipitation and removal from wastewater using a microfilter device. In that case, however, separate determinations of appropriate dosage must be made for the coagulant and the precipitant due to changing levels of dispersancy and metals loading. While coagulant products may improve flux, the consistency of such performance will be poor, i.e., there will be episodes of diminished permeate flux and quality. The advantage to using polymers containing DTC functionalities is the enhanced performance of membrane units relative to permeate flux and permeate quality, as well as the consistency of such performance with changing wastewater composition. Also, the quantity of chemical used and, ultimately, chemical sludge produced, will be greatly diminished with the DTC-polymer relative to DMDTC and coagulant.